1. Field of the Invention
The present invention is related to a system for detecting the position of a moving body, and particularly to a system for detecting the position of a moving body or moving vehicle such as a mobile machine used for agricultural or civil engineering works, or an automatic transporting apparatus used in a factory.
2. Description of the Prior Art
A previously proposed system for detecting the current position of a moving body as described above comprises means for circularly scanning a light beam generated in a moving body around the moving body, retroreflector means fixed at at least three positions spaced apart from the moving body for reflecting light in the direction of incidence, and light beam receiver means for receiving the light reflected by the retroreflector means (Japanese Patent Application Laid-Open Publication No. 67476/1984).
In this system, the differential azimuths between two of the three retroreflector means as viewed from the moving body are detected on the basis of the outputs of the light beam receiver means. The position of the moving body is calculated based upon the detected azimuths and information (positional information) representing the preset positions of the individual retroreflector means.
In the above-mentioned system, there is a possibility that the moving body or moving vehicle travels while it is inclined or it jolts during the traveling, whereby it cannot apply the light beam emitted from the moving vehicle to retroreflector means and thus the light beam receiver means cannot detect the light reflected by the retroreflector means. Further, the light beam receiving means may receive light from a reflecting object other than predetermined retroreflector means. If the light from the retroreflector means cannot be detected or light from the other object is accidentally detected as the reflected light of the predetermined retroreflector means, then it is possible that the position of the moving vehicle cannot correctly be calculated and the moving vehicle cannot be allowed to travel along a predetermined course.
As a countermeasure therefor, the present applicant proposed a following control system (USP. No. 5031101). In this system, on the basis of the azimuthal data of each retroreflector means with respect to the advance direction of the moving vehicle which has already been detected by the current and previous scans of the light beam, the azimuth in which the same retroreflector means is to be detected in the next scan is predicted. And the light which is incident from the predicted azimuth is determined to be the proper light reflected by the predetermined retroreflector means. If no incidence of light from the predicted direction repetitively occurs, the moving vehicle is stopped.
In addition, the present applicant proposed a control system in which, if no light is incident from a predicted direction, the predicted direction data is used instead of the actual azimuth to perform the position detection of the moving vehicle as a temporary substitute measure, on the basis of a judgment that the predetermined retroreflector means must exist in the vicinity of the predicted azimuth (U.S. Pat. No. 5,187,662).
In this control system, if the missing of a retroreflector means is temporary, the predicted azimuth is deemed to be the true azimuth in which the actual retroreflector means exists, without stopping the moving vehicle, and the predicted azimuth data is used to detect the position of the moving vehicle. As long as the missing of the retroreflector means is temporary, the error between the predicted and actual directions is small, and thus such measure provides no hindrance in practical use. However, retroreflector means can be lost frequently or for a long time depending on the surface condition of the road on which the moving vehicle is traveling, and a new counter measure will be required in such case.
On the other hand, a counter measure before the missing of retroreflector means rather than the above-mentioned one to be taken after the missing of retroreflector means is also considered. For instance, to allow a light beam to successfully be applied to retroreflector means, a light beam scanning apparatus was proposed in which the generated light beam is horizontally scanned while it is vertically vibrated at a high speed (frequency) by a galvano mirror or polygon mirror (Japanese patent application Laid-Open Publication No. 242313/1985).
FIGS. 25A and 25B show scan loci (tracks of light) of a light beam produced by such conventional apparatus. FIG. 25A shows a part of a light track when the light beam is subjected to a vertical vibration scan by a galvano mirror while it is rotatively scanned in the horizontal direction. FIG. 25B shows a part of the light track when the vertical vibration scan of the light beam is performed at a very high speed by a polygon mirror.
The above-mentioned light beam scanning apparatus has the following problems.
In the system wherein the light beam projected from the moving vehicle is also subjected to a vertical vibration scan with a predetermined amplitude using a galvano mirror, so that the light beam can be applied to the retroreflector means with a probability as high as possible even if the moving vehicle travels on the inclined road surface or it jolts, the amplitude of the light track at the retroreflector means becomes larger and the wavelength thereof or the gap between two adjacent light tracks becomes longer as the distance between the moving vehicle and the retroreflector means increases. For this, it may occur that the retroreflector means 6 cannot intersect the light beam track as shown in FIG. 25A, for example.
Also in the system utilizing a polygon mirror, the distance between any two adjacent light tracks at the retroreflector means 6 widens as the distance between the moving vehicle and the retroreflector means 6 increases. For this, it may occur that the retroreflector means 6 and the light beam cannot intersect as shown in FIG. 25B for example. It is required to increase the ratio of the vertical vibration speed to the horizontal scanning speed, or more particularly, to increase the vertical vibration speed and/or decrease the horizontal scanning speed for decreasing the wavelength of the light track or an interval between two adjacent light tracks, so that the light beam will more surely be incident on the light reflector means.
However, increasing the driving speed of the galvano mirror or polygon mirror is very difficult because of mechanical restrictions. If the scan speed in the horizontal direction is decreased, then the number of data received per minute decreases and the position detection precision degrades, and particularly reduction in detection precision is undesirably significant in the use for position detection of a moving body such as a moving vehicle.
Further, in the above described systems, the reflecting surface of the galvano- or polygon mirror relatively inclines with respect to the light beam generator or receiver means. Accordingly, there is a problem that it is required to increase the reflecting surface area of the reflecting mirror at least more than that corresponding to optical axis deviation due to the inclination, and the light beam flux expands accordingly to reduce the intensity of the light beam.